1. Field of the Invention
The present invention relates generally to semiconductor laser devices, and more particularly to a distributed feedback (DFB) semiconductor laser device having high optical output power, narrow spectral linewidth, and excellent single-longitudinal mode lasing characteristics, and which is capable of being manufactured at high product yield.
2. Discussion of the Background
With the recent demand for increased bandwidth for data communications, optical networks and the components essential for their operation are being closely studied. To provide a light source for such optical networks, semiconductor laser devices such as the distributed feedback semiconductor laser device have been used. FIG. 8 shows a cross section of an exemplary distributed feedback semiconductor laser 800 (hereinafter, referred to as a DFB laser). As seen in this figure, the DFB laser has an active layer 801 wherein radiative recombination takes place, and a diffraction grating 803 for changing the real part and/or the imaginary part of the refractive index (complex refractive index) periodically, so that only the light having a specific wavelength is fed back for wavelength selectivity. The diffraction grating 803 is comprised of a group of periodically spaced parallel rows of grating material 805 surrounded by a cladding material 807 (typically made of InP material) to form a compound semiconductor layer that periodically differs in refractive index from the surroundings. In a DFB laser having such a diffraction grating 803 in the vicinity of its active layer 801, the lasing wavelength XDFB which is emitted from the DFB laser is determined by the relation:
xcexDFB=2neffxcex9,
where xcex9 is the period of the diffraction grating as shown in FIG. 8, and neff is the effective refractive index of the waveguide. Thus, the period xcex9 of the diffraction grating and the effective refractive index neff of the waveguide can be adjusted to set the lasing wavelength xcexDFB independent of the peak wavelength of the optical gain of the active layer.
This setting of the lasing wavelength XDFB independent of the peak wavelength of the optical gain of the active layer allows for essential detuning of the DFB laser device. Detuning is the process of setting the emitted lasing wavelength of a laser to a different value than the peak wavelength of the optical gain of the active layer to provide more stable laser operation over temperature changes. As is known in the art, a moderately large detuning value (that is, a large wavelength difference between the emitted lasing wavelength and the peak wavelength of the optical gain of the active layer) can improve high speed modulation or wide temperature laser performance, while too large a detuning amount degrades performance. The present inventors have recognized that the amount of detuning changes over wide temperature range because temperature dependence on the lasing wavelength is about 0.1 nm/C, while the gain peak wavelength changes at about 0.4 nm/C. Thus, for wide temperature operation, reduction of optical gain especially in the high temperature range should be considered carefully in designing the detuning.
In addition to detuning, the lasing wavelength xcexDFB may also be set independent of the peak wavelength of the optical gain of the active layer in order to the obtain different characteristics of the semiconductor laser device. For example, when the lasing wavelength of the DFB laser is set at wavelengths shorter than the peak wavelength of the optical gain distribution, the differential gain increases to improve the DFB laser in high-speed modulation characteristics and the like. Where the lasing wavelength of the DFB laser is set approximately equal to the peak wavelength of the optical gain distribution of the active layer, the threshold current of the laser device decreases at room temperature. Still alternatively, setting xcexDFB at wavelengths longer than the peak wavelength improves operational characteristics of the DFB laser, such as output power and current injection characteristics, at higher temperatures or at a high driving current operation.
The conventional DFB laser such as that disclosed in FIG. 8 can be broadly divided into a refractive index coupled type laser and a gain coupled type laser. In the refractive index coupled DFB laser, the compound semiconductor layer constituting the diffraction grating has a bandgap energy considerably higher than the bandgap energy of the active layer and the bandgap energy of the lasing wavelength. Thus, bandgap wavelength (which is a wavelength conversion of the bandgap energy) of the diffraction grating is typically at least 100 nm shorter than the lasing wavelength and is usually within the range of 1200 nm-1300 nm if the xcexDFB is approximately 1550 nm. In the gain coupled DFB laser, the bandgap wavelength of the compound semiconductor layer constituting the diffraction grating is longer than the lasing wavelength and is typically about 1650 nm if the xcexDFB is approximately 1550 nm. FIGS. 9a and 9b show the operational characteristics of an exemplary refractive index coupled laser and gain coupled laser respectively. Each of these figures includes xcexe, xcexg, xcexmax, and xcexInP shown plotted on an abscissa which shows wavelength increasing from left to right in the figures. In this regard, xcexe is the selected lasing wavelength of the DFB laser 800, xcexmax is the peak wavelength of the optical gain distribution of the active layer 801, xcexg is the bandgap wavelength of the diffraction grating material 805, and xcexInP is the bandgap wavelength of the surrounding InP material 807. As seen in FIGS. 9a and 9b, the bandgap wavelength xcexInP is typically 920 nm and the bandgap wavelength xcexg is closely related to the absorption loss of the diffraction grating which is shown by the broken curves 903 and 903xe2x80x2. Moreover, the refractive index of a material increases as the bandgap wavelength of the material increases as shown by the arrows 905. Thus, as seen in the figures, the refractive index of the diffraction grating having the bandgap wavelength xcexg is generally higher than the refractive index of the surrounding Inp layer having the bandgap wavelength xcexInP.
FIG. 9a shows an exemplary refractive index coupled DFB laser wherein the DFB laser has a lasing wavelength xcexe of 1550 nm and bandgap wavelength xcexg of 1250 nm, and satisfies the relationship:
xcexg less than xcexe.
Thus, the DFB laser of FIG. 9(a) reflects xcexexe2x88x92xcexg=300 nm. The DFB lasing wavelength xcexe is usually set within the several tens of nanometer range from the peak wavelength xcexmax of the optical gain distribution of the active layer. In the FIG. 9(a), xcexe is located longer than xcexmax. With the refractive index coupled DFB laser, the absorption loss curve 903 does not cross the lasing wavelength xcexe and therefore absorption loss at xcexe is very small. Accordingly, the DFB laser of FIG. 9a, has the advantage of a low threshold current and favorable optical output-injection current characteristics. However, as also shown in FIG. 9a, in a refractive index coupled DFB laser, the absorption loss curve 903 also does not cross the peak wavelength of the optical gain distribution of the active layer xcexmax. Therefore, assuming that the absorption coefficient with respect to the lasing wavelength xcexe of the DFB laser is xcex1e and the absorption coefficient with respect to the bandgap wavelength of the active layer, or the peak wavelength xcexmax of the optical gain distribution of the active layer, is xcex1max, then xcex1e is approximately equal to xcex1max which is approximately equal to zero. This means that the absorption curve 903 affects neither xcexmax nor xcexe, and the peak wavelength xcexmax of the optical gain distribution of the active layer is not suppressed with respect to the lasing wavelength xcexe.
More specifically, there is a problem with the refractive index coupled laser in that a side mode suppression ratio (SMSR) of adequate magnitude cannot be secured between the lasing mode at the designed lasing wavelength xcexe of the DFB laser and the mode around the peak wavelength xcexmax of the optical gain distribution of the active layer. In addition, because neither the xcexmax nor the xcexe wavelengths are affected by the absorption curve 903, wide detuning cannot be accomplished using the refractive index coupled semiconductor laser of FIG. 9a. That is, the absolute value of the detuning amount |xcexe-xcexmax| cannot be made greater since an increase in the absolute value of the detuning amount |xcexe-xcexmax| would result in a large gain difference between the lasing wavelength xcexe and xcexmax, and lowers the single mode properties and narrows the temperature range operation of the refractive index coupled semiconductor laser.
Finally, with the refractive index coupled DFB laser of FIG. 9a, the difference in the refractive index of the grating material 805 and the refractive index of the InP buried layer 807 is relatively small. Therefore, the physical distance between the grating material 805 and the active layer 801 of the DFB laser 800 must be reduced and, as a result, the coupling coefficient varies greatly depending on the thickness of the diffraction grating layer and the duty ratio which is expressed as W/xcex9, where W is the width of one element of the diffraction grating and xcex9 is the pitch of the gratings. This makes it difficult to fabricate refractive index coupled DFB laser devices having the same characteristics resulting in low manufacturing yields for this type of laser.
As seen in FIG. 9b, the gain coupled DFB laser has a lasing wavelength xcexe of which is less than the bandgap wavelength xcexg of the diffraction grating layer. Specifically, the DFB laser of FIG. 9b has a lasing wavelength xcexe of 1550 nm, a bandgap wavelength xcexg of 1650 nm, and satisfies the relationship:
xcexe less than xcexg.
Thus, this exemplary DFB laser reflects xcexexe2x88x92xcexg=xe2x88x92100 nm. in the gain coupled DFB laser of FIG. 9b, there is a relatively large difference between the refractive index of the grating material 805 and refractive index of the InP buried layer 807 which makes it possible to increase the distance between the grating material 805 and the active layer 801. As a result, unlike the refractive index coupled DFB laser, the coupling coefficient of the gain coupled laser is hard to vary with the thickness of the diffraction grating layer and the duty ratio, and same-characteristic DFB lasers can be fabricated with stability thereby allowing higher production yields for this type of laser.
However, as also seen in FIG. 9b, the gain coupled DFB laser has an absorption loss curve 903xe2x80x2 that crosses the lasing wavelength xcexe and, therefore, absorption loss at the desired lasing wavelength xcexe is large resulting in a high threshold current and unfavorable optical output-injection current characteristics. Moreover, although the absorption loss curve 903xe2x80x2 also crosses the undesired wavelength of xcexmax, the absorption coefficient xcex1max is approximately equal to the absorption coefficient xcex1e. That is, as with the refractive index coupled DFB laser, the absorption curve 903xe2x80x2 of the gain coupled DFB laser affects xcexmax and xcexe equally and the peak wavelength xcexmax of the optical gain distribution of the active layer is not suppressed with respect to the lasing wavelength xcexe resulting in a low side mode suppression ratio (SMSR). For example, in the conventional DFB lasers of FIGS. 9a and 9b, the SMSR, though depending on the amount of detuning to the lasing wavelength of the DFB laser, falls within a comparatively small range of 35 and 40 dB. Also like the refractive index coupled DFB laser, since the absorption curve 903xe2x80x2 affects xcexmax and xcexe equally, wide detuning cannot be accomplished because the wider the spacing between the xcexmax and xcexe wavelengths, the smaller the gain of the desired lasing wavelength xcexe will be with respect to the undesired xcexmax. Thus, whether the xcexe is set shorter or longer than xcexmax, the absolute value of the detuning amount |xcexexe2x88x92xcexmax| of conventional refractive index and gain coupled DFB lasers is limited several tens of nanometers thereby causing unfavorable single mode and temperature range characteristics for these devices.
U.S. patent application Ser. No. 09/906,842, the entire contents of which is incorporated herein by reference, discloses a DFB laser device having a selective absorption characteristic that enhances single longitudinal mode operation of a DFB semiconductor laser device over a relatively wider detuning range. The present inventors have recognized, however, that the ever increasing need for greater power from a DFB laser will require a demand for longer cavity length DFB lasers. The present inventors have also discovered that increasing the cavity length to increase the output power of the laser will generally diminish the overall operational characteristics of the DFB laser.
Accordingly, one object of the present invention is to provide a semiconductor laser device and method which overcomes the above described problems.
Another object of the present invention is to provide a DFB laser device having an increased cavity length for providing higher power, and also having good overall operational characteristics.
Another object of the present invention is to provide a semiconductor laser device having high optical output power, narrow spectral linewidth, and excellent single-longitudinal mode lasing characteristics, and which is capable of being manufactured at high product yield.
According to a first aspect of the invention, a semiconductor laser device and method for providing a light source are provided. The device on which the method is based includes a resonant cavity with a cavity length, an active layer structure provided within the resonant cavity and configured to radiate light in an optical gain distribution having a peak wavelength, an embedding layer provided within the resonant cavity and having a refractive index, and a diffraction grating embedded within the embedding layer and having a bandgap wavelength and a refractive index, the diffraction grating configured to select an emission wavelength of the resonant cavity independently of the peak wavelength in the optical gain distribution of the active layer structure. The embedding layer and diffraction grating are configured to provide operational characteristics satisfying the relationship 0 less than xcexexe2x88x92xcexgxe2x89xa6100 nm, where xcexe is the emission wavelength of the resonant cavity xcexg is the bandgap wavelength of the diffraction grating. In addition, a difference in the refractive index of the diffraction grating and the embedding layer satisfies the relationship 0.29 less than n1xe2x88x92n2, where n1 is the refractive index of the diffraction grating and n2 is the refractive index of the embedding layer.
The embedding layer and diffraction grating may be configured to provide operational characteristics satisfying the relationship 0.8 less than xcexaL less than 2.0, where xcexa is a coupling coefficient of the diffraction grating and L is the length of the cavity. Alternatively, operational characteristics satisfying the relationship 1.2xe2x89xa6xcexaL less than 2.0; 1.0xe2x89xa6xcexaL less than 2.0; or 0.8 less than xcexaL less than 1.2 may be provided. Moreover, the cavity length may be at least 300 xcexcm and the laser device may include an active layer structure having a strain-compensated quantum well structure. In one embodiment, the strain-compensated quantum well structure includes six quantum well layers each having a thickness of about 5 nm, and six barrier layers associated with the quantum well layers and each having a thickness of about 10 nm. Each of the quantum well layers has a compression strain approximately within the range of 0.8%-1.2%, preferably 1.0%, and each of the barrier layers has a tensile strain approximately within the range of 0.1%-0.4%, preferably 0.1%.
The DFB laser has a coupling coefficient with a variance of less thanxc2x110 cmxe2x88x921, preferably approximatelyxc2x13 cmxe2x88x921. Moreover, the active layer of the DFB laser is preferably configured to provide the following operational characteristics:xe2x88x9220 nm less than xcexexe2x88x92xcexmax less than 0 nm, where xcexmax is the peak wavelength in the optical gain distribution of the active layer. Alternatively, the operational characteristics:xe2x88x9220 nm less than xcexexe2x88x92xcexmax less than xe2x88x9210 nm, may be provided.
In another aspect of the invention, a laser module having a semiconductor laser device coupled to an optical fiber is provided. The semiconductor laser device includes a resonant cavity with a cavity length, an active layer structure provided within the resonant cavity and configured to radiate light in an optical gain distribution having a peak wavelength, an embedding layer provided within the resonant cavity and having a refractive index, and a diffraction grating embedded within the embedding layer and having a bandgap wavelength and a refractive index, the diffraction grating configured to select an emission wavelength of the resonant cavity independently of thepeak wavelength in the optical gain distribution of the active layer structure. The embedding layer and diffraction grating are configured to provide operational characteristics satisfying the relationship 0 less than xcexexe2x88x92xcexgxe2x89xa6100 nm, where xcexe is the emission wavelength of the resonant cavity xcexg is the bandgap wavelength of the diffraction grating. In addition, a difference in the refractive index of the diffraction grating and the embedding layer satisfies the relationship 0.29 less than n1xe2x88x92n2, where n1 is the refractive index of the diffraction grating and n2 is the refractive index of the embedding layer.